The Domino Crown can be found on the first floor of the Clothing Store and purchased for 420 Bucks. Purchasing this hat will allow you to wear this hat only in Adopt Me!, unless you buy this item from the Roblox Catalog.
The description of this item on the Roblox Catalog reads
"This golden crown was awarded as a prize in the June 2007 Domino Rally Building Contest. Perhaps its most unique characteristic is its ability to inspire viewers with awe while at the same time making the wearer look goofy."